1. Field of the Invention
The invention refers to a process and apparatus for drying a slurry, particularly a sludge, such as sewage sludge, in which a sludge mixture of recycled dried sludge and wet sludge is fed to a drier to dehydrate the sludge mixture utilizing hot exhaust air from a furnace. The quantity of wet sludge or recycled dried sludge fed to the drier is controlled based upon the drier inlet temperature.
2. Description of the Prior Art Processes for drying sludge have been known in the art, for example, as described in WO 93/24800 or U.S. Pat. No. 5,069,801. In these processes, the furnace temperature and the drier inlet temperature, respectively, are controlled in response to changing dry content levels in the sludge mixture. As a result, the drier inlet temperature has to be lowered when the dry solids content of the sludge mixture fed into the drier increases (i.e., when the sludge mixer fed into the drier contains less water). This causes a decrease in drying performance since the drier inlet temperature is lower. If a wetter sludge mixture is fed into the drier, the drier inlet temperature has to be increased. However, the furnace is limited in capacity, and when the furnace reaches its upper limit, adequate drying can only be achieved by reducing throughput, which usually must be effected by manual intervention.
This invention addresses these problems in the art by providing a process and apparatus in which the operating conditions of the drier are kept substantially constant by using a control system according to the present invention.